Add an image texture to a shader subgraph
Unfortunately there isn't any way to create a NodeGroup that has a Texture selection as an input without some coding to create Custom nodes. For the Custom node, you'd have to have the image texture node in the node tree, whose value you would change via code using the custom node UI. Please read information in the link below for a much more thorough explanation of this process.
Cycles Materials Math node with more than two inputs